I think generally paying for a messaging app subscription more than you pay for your phone subscription (ARPU) is going to be hard to sell to a wide audience.
I tend to agree with this. I understand the convenience it offers, but as long as the services it connects to are "free", I'm not going to pay more than one or two dollars / euros for it. This is not being stingy, simply a subjective evaluation of the worth of such a service.